I used to run CyanogenMod 9.1 with device encryption on my Galaxy S3, and decided today that I wanted to try a CM 10 nightly.

A problem with device encryption that I didn't know about before is that it's impossible to turn off. ClockworkMod Recovery (5.8.4.3) can't mount the /data partition (and, by extension, /sdcard). This means that I can't use CWM to factory reset or install a new update.zip, since the update.zip must lie on the internal sdcard.

Trying "choose zip from sdcard" gives "E:Can't mount /sdcard/" and choosing "apply /sdcard/update.zip" gives "E:Can't mount /sdcard/update.zip". This happens even if I've used adb to put the update.zip in the /sdcard mountpoint.

Factory resetting from CWM gives these two errors: "Error mounting /data!" and "Error mounting /sdcard/.android_secure!". Factory resetting from CM9 didn't work. I've since tried to flash CM10 with MobileOdin, which didn't work, and now I can only boot to CWM.

Does anyone have any idea about how I could mount /data or otherwise get a working ROM?

I've taken backups of everything. The tools I suppose could help me, but I haven't tried because I'm new at this and don't want to hard-brick my phone, are fastboot and heimdall.

TL;DR: S3 only boots to CWM, but CWM won't mount /data and so can't flash zips.